1. Field of the Invention
The present invention relates to a wind turbine with a primary and a secondary generator, a method of retrofitting a wind turbine having a primary generator to also have a secondary generator, and method of operating such wind turbines.
2. Description of Related Art
In particular, the present invention relates to a wind turbine of variable speed type comprising: a wind turbine tower; a nacelle provided on said wind turbine; a wind turbine rotor hub rotatably mounted at said nacelle, said wind turbine rotor hub having at least one wind turbine blade mounted thereon and a shaft coupled to said wind turbine rotor hub and to, optionally via a gear box, a primary generator which via power lines has a primary stator electrically connected to a grid connection and a primary rotor electrically connected to a back-to-back converter at a generator side converter end and wherein the back-to-back converter at a grid side converter end is electrically connected to the grid connection. The wind turbine further comprises a secondary generator coupled to the shaft via a mechanical coupling and electrically connected to the primary rotor of the primary generator and to the generator side converter end of the back-to-back converter.
The background of the invention is a hybrid power-generating device, known from U.S. Pat. No. 7,518,257 B2 which discloses a wind turbine with a primary electrical generator and an auxiliary generator with different characteristics. The primary generator is a double-feed induction generator (DFIG) configuration while the secondary generator is a synchronised generator (SG) configuration. The primary generator is coupled to a first generator side of a back-to-back converter while the secondary generator is coupled to a second generator side of the back-to-back converter. The secondary generator is coupled to the DC link in the converter and is used to supply power to the grid side of the converter in the event of a fault in the drive train or a power drop in the grid. This configuration has the drawback that it requires the use of two different generator sides each designed to match the power range of a particular generator type. This in turn increases the complexity and number of components of the power converter. This configuration does not provide means for a smooth transition when the DFIG generator is switched in which may introduce power or moment spikes in the drive train resulting in faults during operation.
It is well known that permanent magnet generators are expensive and that full scale converter wind turbine systems are expensive.
One well known solution to this is a DFIG configuration where stator of the generator is connected directly to the grid, and the rotor is connected to a back-to-back converter of a smaller size. This configuration is well known and advantageous in that it in comparison with a full scale converter configuration only requires a converter that is about one third of the size of the converter needed in a full scale configuration where all the power is converted. Hence, the converter becomes smaller, more effective and much more cost effective.
However, the DFIG configuration has a drawback in that a wind turbine does not produce electricity at low wind speeds.
In particular, off the shelf DFIG-systems exist and are readily available in the MW-power range.
Generators with permanent magnets (PM) exist and are advantageous, because they do not need exciter power, and hence in wind turbines have a relatively high efficiency at all wind speeds. However, PM-based generators are expensive due to the high costs for systems in the MW-power range.